I am having trouble being able to save my SolidWorks file as a Composer file without having missing parts. I have an assembled elevator model, so it has a lot of components in it. When I open the file in Composer it is missing a lot of parts from the assembly. Is it because there are too many parts? Or did something new happen to the 2018 version? This never happened when I had the 2017 version.

Typically this happens when Large Assembly Mode/ Lightweight mode is active.

Before opening the file If you have SOLIDWORKS installed on the machine turn this off under System Options > Assemblies and also make sure than under System Options Perfomance "auotmatically load components lightweight" is also turned off.

This was always reccomened but became more important with changes that were made during the 2018 release. If you don't have SOLIDWORKS installed then it is a bit more complicated as it needs changes in the Windows registry